The process of preparing optical or ophthalmic lenses begins with an unfinished or semi-finished glass or plastic lens blank. Typically a semi-finished lens blank has a finished polished front surface and an unfinished back surface. By grinding away material from the back surface of the lens blank the required corrective prescription is generated. Thereafter the surface having had the corrective prescription imparted thereto is polished and the peripheral edge of the thus processed lens blank is provided with a final desired contour thereby establishing a finished optical or ophthalmic lens. The lens blank can be either a plastic or a glass lens blank.
During these various processing operations the lens blank is securely maintained in accurate alignment and in place on a lens holding block. This procedure is often referred to as “lens blocking”.
Various materials are employed to secure the lens blank to the lens holding block. These materials include glues, pitch and low temperature fusible metal alloys. The use of glues and pitch, in addition to being messy, suffers the further disadvantage of generally being non-reusable or non-reclaimable. While the prior art use of low temperature metal alloys eliminated some of these disadvantages experienced with the use of glues and pitch, nonetheless, the use of these metal alloys, both in their preparation and their reclamation caused significant environmental and health hazards especially since these alloys were most often fabricated from such metals as cadmium, tin, lead and bismuth. Of these metals, lead and cadmium are the most toxic. Lead is strong protoplasmic poison and can be introduced into the body by ingestion, inhalation and skin absorption. Cadmium poisoning is similar to lead in many ways and is introduced into the body in the same way as lead. Like lead, cadmium is stored in the liver, kidney and bone. Procedures for the formulation of such alloys and reclamation processes so as to enable its re-use as a material to secure a lens blank to a lens holding block thus exposes workers to serious environmental and health hazards. The formation of these alloys often is accomplished through the use of powdered or particulate metals which are subjected to a sintering and heat treating process. Fumes and/or dust particles of these metals are released to the ambient atmosphere thereby creating environmental and health hazards for those formulating these alloys. The same hazards exist for those attempting to reclaim the used low temperature metal alloy blocking material.
To overcome these issues, organic low shrinkage materials have been developed to be used as lens blocking materials.
U.S. Pat. No. 6,036,313 in the name of 3M Innovative Properties Company discloses examples of compound families suitable for lens blocking with thermoplastic materials.
The disclosed blocking compositions have many advantages over traditional metal alloy materials. For example, the lens blocking compositions are non-toxic, environmentally safe, and preferably biodegradable. The materials preferably can be used with existing processing equipment and may be recycled. An ophthalmic lens holding block can be used that comprises a solidified mass of a thermoplastic blocking composition. The blocking composition may comprise a homopolymer or copolymer of epsilon-caprolactone, and preferably has a number average molecular weight of at least 3,000, a mean bending modulus of at least 69 MPa at 21° C., or a mean flexural strength of at least 1 MPa at 21° C. The composition is solid at 21° C. and has a sufficiently low melting or softening point such that the composition may be placed adjacent to an ophthalmic lens blank while at its melting or softening point without damaging the lens blank. The composition also has sufficient adhesion to a lens blank or to a lens blank coating or tape to hold an ophthalmic lens during a generating procedure.
U.S. Pat. No. 6,036,313 discloses a method of holding an ophthalmic lens blank, comprising the steps of:                providing a lens blocking composition as described above;        heating the lens blocking composition to its melting or softening point;        providing a blocking material receiving cavity against the lens blank;        forming the ophthalmic lens blocking composition into the receiving cavity; and allowing the composition to solidify.        
Alternatively, a method of holding an ophthalmic lens blank is described, comprising the steps of:                providing an ophthalmic lens block comprising a solidified mass of a thermoplastic blocking composition, and preferably comprising a heat absorbing material;        heating the surface of the lens blocking composition to its melting or softening point;        positioning a lens blank against the softened surface of the lens blocking composition; and allowing the composition to resolidify.        
However the inventors have noticed that separating the thermoplastic blocking composition from the lens blank or from the lens blank tape or coating is an issue. This deblocking step comprises ordinarily a hammering step and a crack is generated at the interface between the thermoplastic blocking composition and the lens blank or the lens blank tape or coating.
Said step needs an operator, is often made after separating the ophthalmic lens block from the grinding machine, is time consuming and may introduce defects on the final lens.